PM8805 datasheet(33/48 Pages) STMICROELECTRONICS

33 / 48 page

DS12813 Rev 2

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PM8805

PM8805 overview

4.12

Type T0, T1, T2 output signals

With 3 signals, T0, T1 and T2, 8 different Types of PSE-PD systems are advertised to the

PD system:

– Type 1 and 2 as currently specified in the IEE802.3-2012 standard;

– Type 3 and 4 are the new types introduced with the new IEEE802.3bt 4PPoE

standard;

– Legacy 4-pair are PSEs which perform Type1 or Type2 classification, then power up

the PD over 4-pairs. They may also be new Type3 PSEs limited to Type1 or Type2

power levels. To properly detect 4-pair legacy applications PM8805 embeds a 4P

detection circuit which monitors how many bridges are powered.

– Auxiliary power source such as wall adapter.

All the output configurations are described in Table 11, and are activated when the VDD

voltage exceeds Vuvlo_rising, or an auxiliary source configuration is detected.

The 3 signals are open drain, with 6 V rating, and in most of the cases can be pulled up to

VB reference voltage with a resistor in the range of 47 k to 100 kOhms.

Depending on the information needed by the system, it is possible to combine the outputs in

order to minimize the number of signals transmitted. This is especially worthwhile in isolated

application, where expensive optocouplers are needed to pass the information between

primary and secondary side.

As an example, if a Type3 51 W PD just needs to know if the PSE allocated class matches

with its requested class, it may just monitor T1. If T1 is set to 0, the PD knows that the PSE

Table 10. PM8805 control signals description table

Input signals

Output signals

Standard operations

FAUX

RAUX

STBY

PGD

Hot swap

Charge

pump

Active

bridge

MPS

Normal POE operation

1 after

85ms hot

swap

enabled

Closes @

UVLO

On @

UVLO

Enabled

Off

Standby PoE operation

1 when

hot swap

closed

Closes @

UVLO

Off

LS enabled

HS OFF

Front Aux operation

1 when

hot swap

closed

Closed

Enabled

Off

Rear Aux operation

Open

Off

Additional operations

Sleep mode/

Wake Up

Open

Off

LS enabled

HS OFF

Rear Aux with MPS

Open

Off

LS enabled

HS OFF

Latched shutdown

Open

Off

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